In order to keep down the amount of emission of carbonic acid gases from an automobile, high strength steel sheet is being used to try to reduce the weight of the automobile body. Also, in order to ensure the safety of passengers as, high strength steel sheet is now being frequently used in addition to mild steel sheet for the automobile body. Further, in order to reduce the weight of the automobile body in the future, there is a fast growing new demand for raising the level of usage strength of high strength steel sheet more than the past.
However, when bending high strength steel sheet, due to the high strength, the bent shape tends to depart from the shape of the die and return to the shape before bending. The phenomenon of trying to return to the original shape even after bending is referred to as “springback”. When this springback occurs, even if the steel sheet is bent, the intended shape cannot be obtained in the bent part after bending.
Also, a wall camber phenomenon arises wherein the flat surface of a side wall becomes a surface having curvature due to elastic recovery from bending and springback during shaping. The intended shape cannot be obtained in the bent part and poor dimensional accuracy occurs.
Accordingly, in the body of a conventional automobile, steel sheet of a high strength of 440 MPa or less has been mainly used.
Irrespective of the fact that the weight of automobile bodies must be reduced by using steel sheet of a high strength of 490 MPa or more for the automobile body, in actuality a high strength steel sheet having small springback and good shape fixability does not exist.
There is no need to add that it is extremely important to increase the shape fixability after shaping in steel sheet of a high strength of 440 MPa or less and mild steel sheet for improving the shape accuracy of products such automobiles and home electric appliances.
JP-A-10-72644 discloses an austenitic stainless cold-rolled steel sheet having a small springback (dimensional accuracy in the present invention) characterized in that the degree of integration of the {200} structure in planes parallel to a rolled surface is 1.5 or more. The publication, however, does not describe any art for reducing the springback phenomenon or the wall camber phenomenon of ferritic steel sheet.
Also, as art for reducing the springback of the ferritic stainless steel, JP-A-2001-32050 discloses an invention setting a reflected X-ray intensity ratio of {100} planes parallel to the sheet surface at 2 or more at the center of the sheet thickness. This publication, however, does not describe anything concerning the reduction of the wall camber and does not describe anything regarding the group of {100}<011> to {223}<110> orientations and the {112}<110> orientation important for the reduction of the wall camber.
Also, some of the present inventors disclosed a thin ferritic steel sheet having a ratio of the {100} planes and {111} planes of at least 1 for the purpose of improvement of the shape fixability in the WO00/06791 pamphlet, but this pamphlet does not describe anything regarding the values of X-ray random intensity ratios of the group of {110}<011> to {223}<110> orientations and {554}<225>, {111}<112>, and {111}<110> as in the present invention.
Also, some of the present inventors disclosed a cold-rolled steel sheet having a reflected X-ray intensity ratio of the {100} planes parallel to the sheet surface of 3 or more and a small springback in JP-A-2001-64750. However, this cold-rolled steel sheet is characterized by defining the reflected X-ray intensity ratio of the {100} plane at the outermost surface of the sheet thickness. The measurement position of the X-rays is different from the mean X-ray intensity ratio of the group of {100}<011> to {223}<110> orientations at “½ t of sheet thickness” defined in the present invention.
Also, the publication does not describe anything at all regarding the {554}<225>, {111}<112>, and {111}<110> orientations.
Also, JP-A-2000-297349 discloses a hot-rolled steel sheet having an absolute value of an in-plane anisotropy Δr of the r value of 0.2 or less as a steel sheet having good shape fixability. However, this hot-rolled steel sheet is characterized in that the shape fixability is improved by lowering the yield ratio. The publication does not describe the control of the texture aimed at improvement of the shape fixability based on the concept explained in the present invention.
On the other hand, stretch flangeability is also an indispensable characteristic when working steel sheet into automobile parts or the like. If the shape fixability of a high stretch flanging steel sheet is improved, the range of application of the high strength steel sheet to an automobile body becomes further wider.
None of the above publications, however, describes anything from the viewpoint of achieving both stretch flangeability and shape fixability.
Also, on the other hand, high strength steel sheet is also required to have a good press formability enabling press forming to automobile parts having complex shapes. As the method of improving the press formability of high strength steel sheet, for example, JP-A-6-145892 proposes a method of leaving at least a certain amount of austenite in the steel and utilizing working-induced transformation from this remaining austenite to martensite. In such a good workability high strength steel sheet, however, the method of improving the shape fixability has not been clarified.
Further, for the method of increasing the absorption of impact energy at the time of collision of an automobile while maintaining a good workability, for example JP-A-11-080879 proposes a method of similarly utilizing residual austenite. In a high strength steel sheet having good workability and absorption of impact energy, however, the method of improving the shape. All cited references are hereby incorporated herein by reference in their entireties.